The present invention relates to an electromagnetic coupling device such as an electromagnetic clutch and an electromagnetic brake and, more particularly, to the mounting structure of a surge voltage absorbing member connected to an exciting coil.
Conventionally, a surge voltage absorbing member is connected to an exciting coil used in an electromagnetic clutch, an electromagnetic brake, or the like to absorb the surge voltage generated when the power supply is shut off. Conventional surge voltage absorbing members include a diode.
Conventional electromagnetic coupling devices including surge voltage absorbing diodes of this type are disclosed in, for example, reference 1 (Japanese Utility Model Publication No. 58-8998), reference 2 (Utility Model Registration No. 2507943), reference 3 (U.S. Pat. No. 5,138,293), and reference 4 (Japanese Utility Model Publication No. 2-38105).
A field core accommodates the surge voltage absorbing diode of the electromagnetic clutch disclosed in reference 1. An annular groove is formed in this field core so as to be positioned on the same axis as that of a rotor. This annular groove accommodates an exciting coil body formed by a winding and the diode.
Lead wires are respectively connected to the start and end of the winding of the exciting coil body. These lead wires are guided outside the field core through its outer peripheral wall. The diode is connected in parallel to the exciting coil body via the lead wires.
The exciting coil body and the diode are insulated and fixed inside the annular groove by injecting a synthetic resin material with insulating properties (to be simply referred to as a casting resin hereinafter) into the annular groove and hardening the resin.
The surge voltage absorbing diode of the electromagnetic clutch disclosed in reference 2 is mounted in an annular coil bobbin stored in an exciting coil body. This coil bobbin is inserted in the annular groove of a field core. That is, the diode disclosed in reference 2 is provided inside the field core together with the exciting coil body.
The coil bobbin has a storage portion in which the exciting coil body is stored, a storage portion in which the diode is stored, and grooves and holes for the inflow of a casting resin into the storage portions. The exciting coil body and the diode are insulated and fixed in the annular groove by injecting a casting resin into the annular groove and hardening the resin while the lead wires of the exciting coils are guided outside the field core.
The surge voltage absorbing diodes of the electromagnetic clutches disclosed in references 3 and 4 are provided outside the field cores.
The field core disclosed in reference 3 includes a coil bobbin to hold an exciting coil body. This coil bobbin is formed into an annular shape and loaded in the annular groove of the field core. The coil bobbin is provided with a wiring terminal block. A support fitting for connecting the winding end portions of the exciting coil to lead wires and a diode are mounted on the terminal block. The diode is connected to the winding end portions of the exciting coil and the lead wires on the terminal block.
This terminal block is provided on one end portion of the coil bobbin in the axial direction so as to protrude therefrom and is inserted in the through hole formed in the bottom portion of the annular groove. The diode is mounted on a portion of the terminal block which protrudes outside the field core.
A waterproof cover is placed on the terminal block from the outside of the field core. This cover is fixed to the outer surface of the field core by welding or adhesive bonding.
The surge voltage absorbing diode of the electromagnetic clutch disclosed in reference 4 is connected to the wiring parts of lead wires of an exciting coil which are located outside a field core. This diode is placed along the lead wires and is fixed to the lead wires with a heat shrinkable tube through which these members extend.
The surge voltage absorbing diodes disclosed in references 1 and 2 described above each are placed in the annular groove of the field core at a position near the exciting coil body. The heat generated by the exciting coil sometimes greatly increases the temperature inside the field core. In this case, the heat load on the diode increases. The heat may degrade the diode. That is, the heat may destroy the circuit of a diode element or deform the sealed resin of the package portion which seals the element.
According to each of the field cores disclosed in references 1 and 2, the storage space for the exciting coil body is narrowed by an amount corresponding to the space to store the diode. For this reason, the electromagnetic clutches disclosed in references 1 and 2 are smaller in the number of turns of an exciting coil than other types of electromagnetic clutches having similar sizes, and hence are lower in performance.
In each of the electromagnetic clutches disclosed in references 3 and 4, the surge voltage absorbing diode is placed at a position spaced apart from the exciting coil. For this reason, this diode is not likely to be influenced by the heat generated by the exciting coil. In this electromagnetic clutch, however, since the diode is positioned outside the field core, it is necessary to employ a structure for waterproofing the diode.
The waterproof structure disclosed in reference 3 is configured to fix the waterproof cover covering the diode to the field core so as to prevent water from entering the field core. The waterproof structure disclosed in reference 4 is configured to cover the diode with the heat shrinkable tube and cause the tube to heat-shrink to prevent water from entering the inside. The electromagnetic clutches disclosed in references 3 and 4 each employ the above waterproof structure, and hence the number of assembly steps increases, resulting in a decrease in productivity.